1. Field of the Invention
The present invention relates to an improved circuit for measuring capacitance and, more particularly, to a circuit for accurately measuring relatively small changes in the capacitances of a pair of variable capacitive elements, which includes a detector and other circuitry for measuring the difference of an electrical characteristic of the capacitive elements, while minimizing errors in the output signal due to null shifts and biasing errors of the detector.
2. Description of the Prior Art
Capacitive measuring circuits are generally known in the art. An example of such a circuit is disclosed in U.S. Pat. No. 4,634,965. Such circuits are known to be used in various applications, including certain known types of accelerometers to provide a signal representative of acceleration. In particular, certain known accelerometers are configured with a proof mass suspended by a flexure which deflects in response to accelerations along a sensitive axis, generally perpendicular to the plane of the flexure hinge axis and perpendicular to the plane of the proof mass. At rest, the proof mass is suspended equidistantly between upper and lower excitation rings. Electrically conductive material, known as pick-off capacitance plates, are disposed on opposing sides of the proof mass to form capacitive elements with the excitation rings. More particularly, a pick-off capacitance plate disposed on the upper side of the proof mass forms a first capacitive element with the upper excitation ring. Similarly, the pick-off capacitance plate disposed on the lower side of the proof mass forms a second capacitive element.
As mentioned above, the proof mass and thus the pick-off capacitance plates are normally equally spaced from the upper and lower excitation rings in an at-rest or null position. An acceleration force along the sensitive axis causes the proof mass to deflect either upwardly or downwardly. Such upward or downward deflection of the proof mass causes a change in the distance between the pick-off capacitive plates and the upper and lower excitation rings. This change in the distance between the pick-off capacitance plates and the upper and lower excitation rings causes a change in the capacitance of the first and second capacitive elements. For example, if the accelerometer is subjected to an upward acceleration, the proof mass will deflect downwardly forcing the distance between the upper excitation ring and the pick-off capacitive plate on the upper side of the proof mass to increase, while the distance between the lower excitation ring and the pick-off capacitive plate on the lower side of the proof mass decreases. Since the capacitance of a capacitive element is a function of the distance between the plates, an upward acceleration would cause a decrease of the capacitance of the first capacitive element and an increase in the capacitance of the second capacitive element. Conversely, accelerations directed downwardly along the sensitive axis will cause the proof mass to deflect upwardly, thus increasing the capacitance of the first capacitive element and decreasing the capacitance of the second capacitive element.
Consequently, the difference in the values of capacitance of the first and second capacitive elements is thus representative of the displacement of the proof mass in response to either an upward or downward acceleration along the sensitive axis. The displacement signal is applied to a servo system that drives electromagnets to return the proof mass to its null position. The magnitude of the drive current to the electromagnets is thus a measure of the acceleration along the sensitive axis.
Since the magnitude of the drive current to the electromagnets is dependent upon the capacitance values of the first and second capacitive elements, it is critical to the accuracy of the system that the capacitance of the capacitive elements be measured as accurately as possible. Unfortunately, known circuits for measuring the capacitance of the capacitive elements, such as disclosed in U.S. Pat. No. 4,634,965, include error sources that contribute to errors in the accelerometer output signal. In particular, the circuit disclosed in the '965 patent includes a pair of junction field effect transistors (JFETs). Such JFETs are known to have different leakage currents and different junction capacitances that can result in errors in the measurement of the capacitance of the first and second capacitive elements which, in turn, result in errors in the output of the accelerometer.
There are other problems with known circuits that can affect the accuracy of an accelerometer. For example, in some known accelerometers, one of the pick-off capacitive plates potentially is able to make physical contact with one of the excitation rings under certain conditions due to the physical configuration of the accelerometer. Such a condition causes a short circuit of one of the capacitive elements that can cause additional errors in the accelerometer for a time period dependent upon the recovery time of the system.